Non-propulsive aircraft systems are typically driven by a combination of different secondary power types such as hydraulic, pneumatic, electrical and mechanical power, which may all be extracted from an aircraft's main engines using various approaches. For example, pneumatic power may be provided by extracting bleed air from various compressor stages of the main engine, and mechanical power may be extracted from a gearbox driven by the output shaft of the main engine. However, extracting energy reduces the engine total efficiency and thereby increases the specific fuel consumption of the main engine. Moreover, the machinery used to extract energy from the main engine may need to be placed within very confined areas of the aircraft, where packaging space is limited. Specifically, the machinery may be placed within the main engine and an associated nacelle that surrounds the main engine. In addition, the machinery may also result in an increased inner to outer wall thickness of the nacelle. Increasing the inner to outer wall thickness will in turn increase the weight and drag associated with the nacelle. The machinery also adds cost and weight to the aircraft as well.
One or more auxiliary power units (APUs) may be included in the aircraft to provide non-propulsive power. However, the APUs are typically treated as a back-up power source, and are infrequently used while the aircraft is in flight. For example, in one approach the APU may serve as an on-line emergency electrical power source in the event a main engine generator or one of the main engines become inoperative.